So far, epoxy resins have been used widely in such fields as various molding materials, adhesives, paints, plywoods and laminates. However, there are problems common to these applications; disadvantageously, cured products are brittle or fragile and, when epoxy resins are used in adhesives, the peel strength is low.
Meanwhile, oxypropylene polymers having a reactive silicon group (a group which is a silicon atom-containing group with a hydroxyl group or a hydrolyzable group being bound to the silicon atom and can form a siloxane bond) have interesting characteristics in that they can be cured at ambient temperature to give rubber-like elastic substances. Generally, however, they are disadvantageous in that the strength of cured products is low, so that their applications are restricted.
For markedly improving the disadvantageous properties of both, namely the brittleness of cured epoxy resins and the insufficient strength of cured oxypropylene polymers, curable resin compositions in which an epoxy resins and a reactive silicon group-containing oxypropylene polymer are combined have been proposed (e.g. Japanese Kokai Patent Publication No. 61-247723 and No. 61-268720).
However, it has so far been difficult to produce oxypropylene polymers having a high molecular weight with a narrow molecular weight distribution (high monodispersity) and accordingly only reactive silicon group-containing oxypropylene polymers having a broad molecular weight distribution (high polydispersity) have been known.
Compositions in which such oxypropylene polymers have a broad molecular weight distribution cause various inconveniences in practical use thereof; for instance, they have a high viscosity and are not easy to handle before curing.
Recently, it has been reported that polyoxypropylenes showing a narrow molecular weight distribution can be produced. See Japanese Kokai Patent Publications Nos. 61-197631, 61-215622, 61-215623 and 61-218632 and Japanese Patent Publications Nos. 46-27250 and 59-15336. The present inventors found that compositions comprising an epoxy resin and a polymer derived from an oxypropylene polymer with a narrow molecular weight distribution, which is used as the main chain, by introducing a reactive silicon group at the terminal of said chain have a low viscosity and are easy to handle before curing and, after curing, give cured products having excellent tensile characteristics and furthermore good chemical resistance and water resistance. This finding has now led to completion of the present invention